Polyethylene terephthalate (PET) is widely used for the production of light weight plastic articles since PET is excellent in mechanical properties such as formability and creep resistance and can be biaxially molecularly oriented. However, during molding or extrusion processes, acetaldehyde is formed by thermal decomposition of the polyester or copolyester containing poly(ethylene terephthalate) units, and when the polyester or copolyester is formed into an article, the acetaldehyde in the article walls migrates into the contents of the article. Small amounts of acetaldehyde adversely affect the flavor retaining property of foods and beverages, and the fragrance retaining property of foods, beverages, cosmetics, and other package contents. For these reasons, it is desirable to minimize the migration of acetaldehyde into package contents.
Acetaldehyde content has been successfully reduced in crystalline PET compositions by solid stating. Solid stating PET not only decreases its acetaldehyde content but also decreases the tendency to form acetaldehyde. Absent solid stating, large amounts of polyamides are required to reduce the level of acetaldehyde to an acceptable range. Such large amounts of polyamides adversely effect the clarity of the polyester blend. Thus, PET which has been solid stated exhibits a much lower acetaldehyde content than PET which has not been solid stated and does not require large amounts of polyamides.
Crystalline PET can be solid stated since crystalline PET has a well defined melting point. In contrast, noncrystalline or amorphous copolyesters can not be solid stated since such copolyesters lack a definite crystal structure and a well defined melting point and thus melt during the solid stating process forming large agglomerates. As a result, noncrystalline copolyesters of PET exhibit higher levels of acetaldehyde than crystalline PET resins. Attempts to reduce the acetaldehyde levels to an acceptable amount in noncrystalline copolyesters by blending large amounts of polyamides produce haze.
The use of polyamides to increase the gas barrier properties in PET based resins is disclosed in U.S. Pat. No. 4,837,115, Japanese Patent Applications (KOKAI) Nos. 62-181336, 62-257959, 62-51442 and 61-276852, and copending commonly assigned U.S. Ser. No. 07/862,027.
U.S. Pat. No. 4,837,115 discloses a polyester/high molecular weight polyamide blend in Which the polyester is a crystalline polyester wherein at least 80 mole percent of the acid component is terephthalic acid and at least 80 mole percent of the diol component is ethylene glycol, and the polyamide is a fatty acid formed by polymerizing an unsaturated fatty acid with a diamine.
Japanese patent Application (KOKAI) No. 62-181336 discloses blending crystalline polyethylene terephthalate resins with up to 30 weight percent of commercial grade high molecular weight polyamide resin.
Japanese Patent Application (KOKAI) No. 62-257959 discloses blending crystalline polyethylene terephthalate resins with 0.1 to 15 weight percent of a high molecular weight polyamide or ethylene/vinyl alcohol copolymer.
Japanese Patent Application (KOKAI) No. 62-51442 discloses a multilayer container having an innermost layer composed of a blend of high molecular weight aliphatic polyamides with a crystalline PET whose acid component is at least 80 mole percent terephthalic acid and whose glycol component is at least 80 mole percent ethylene glycol.
Japanese patent Application (KOKAI) No. 61-276852 discloses blends of polyalkylene terephthalate and high molecular weight polyamides composed of dicarboxylic acid component units that must include saturated hydroxydicarboxylic acid component units.
Copending commonly assigned U.S. Ser. No. 07/862,027, filed Apr. 2, 1992, discloses a blend containing crystalline PET and a low molecular weight polyamide.
In contrast, the present inventor has unexpectedly discovered that nonhydroxyfunctional polyamides used in a critical range of 0.05 to 2.0 weight percent are effective in reducing residual acetaldehyde in essentially noncrystalline copolyesters containing poly(ethylene terephthalate) units without adversely effecting clarity.